In standard frequency-discrimination experiments either the retinal spatial frequencies (cycles per degree) or the object spatial frequencies (real world) could be compared, because the retinal and object frequency differences are the same. Current models of spatial-frequency discrimination assume that observers compare the retinal frequencies. I test this assumption by presenting gratings at different viewing distances (with strong depth cues). The object frequencies of the gratings bear the same relationship that they do in a standard frequency-discrimination experiment, but the retinal frequency of the more distant grating is always markedly higher than that of the near grating. The observer’s task is to compare the object spatial frequencies. This change from one depth to two (with no change in the stimulus object) has a negligible effect on the observer’s performance, suggesting that observers compare object frequencies even in standard spatial-frequency-discrimination experiments. This conclusion is supported by the findings that (1) observers appear unable to learn to compare retinal frequencies and (2) the interstimulus interval has no effect (over the range 0–1020 msec), implying long-term storage of the visual information. Suggestions are made about why these results are consistent with good system design.
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